1. Field of the Invention
The present invention relates to devices for injecting medicaments into human or animal subjects. In particular, the present invention relates to an injector capable of delivering medicaments to animal and human subjects and to a method for delivering medicaments to animal or human subjects.
2. State of the Art
Automatic injectors (hereinafter referred to as “autoinjectors”) are well known in the medical and veterinary industries and enable the automatic injection of a desired dose of medicament to animal or human subjects. Autoinjectors are generally designed according to one of two delivery mechanisms: those that deliver medicament using a needle (hereinafter “needled” autoinjectors); and those that do not (hereinafter “needle-less” autoinjectors). No matter what their design, autoinjectors are thought to exhibit several advantages relative to simple hypodermic syringes. For instance, because autoinjectors may be designed to automatically and reliably deliver a desired dose of medicament on demand, they facilitate quick, convenient, and accurate delivery of medicaments. In particular, autoinjectors are well suited for use by human subjects who must self-administer therapeutic substances. Moreover, where autoinjectors incorporate a needled injection mechanism, they may be designed so that the needle is hidden from view before, during, and after a delivery cycle, thereby reducing or eliminating any anxiety associated with the act of penetrating a visible needle into the subject's tissue. Various different injection devices are disclosed in U.S. Pat. Nos. 3,797,489, 5,176,645, 5,527,287, 5,300,030, and 6,270,479, U.S. patent application 20010005781, and International Publications WO 94/13342 and WO 95/31235.
Despite the benefits they provide, however, state of the art autoinjectors are not generally designed for delivery of viscous medicaments. Because the medicament delivered by needle-less autoinjectors is typically accelerated to a very high velocity (e.g., 800 feet per second (fps) to 1,200 fps) to effect injection, needle-less autoinjectors are not well suited for the delivery of viscous medicaments or medicaments incorporating particles larger than a few microns in any dimension. Moreover, autoinjectors including needled injection mechanisms are generally designed to deliver aqueous solutions having very low viscosities, such as insulin or epinephrine solutions, and, therefore, do not typically address the performance hurdles presented when seeking to deliver viscous medicament via a needled injection mechanism.
Generating an injection force of sufficient magnitude to drive a viscous medicament through a needle of suitable gauge within a suitable amount of time is one performance hurdle that must be overcome in order to deliver a viscous medicament via a needled injection device. To ensure the safety and comfort of the subject, the gauge of the needles used in needled injection devices typically ranges from about 21 gauge to about 31 gauge. Yet, a number of existing and emerging medicaments designed for delivery via subcutaneous, intramuscular, or intra-articular injection exhibit viscosities that range up to and above 10 Poise, 100 Poise, 1,000 Poise, and even 10,000 Poise. As is easily appreciated by reference to the Hagen-Poiseuille Law, F=8QμL(R2/r4), wherein “F” represents the injection force required, “Q” represents the flow rate of the material injected, “μ” represents the viscosity of the material injected, “L” represents the length of the needle used, “R” represents the internal diameter of the reservoir containing the material to be injected, and “r” represents the internal diameter of the needle used, injection of such medicaments through a needle of suitable gauge may require an injection force that approaches or exceeds 100 pounds. For example, the Hagen-Poiseuille Law indicates that in order to deliver 0.5 cc of a medicament having a viscosity of 200 Poise within 10 seconds via a syringe having an internal diameter of 4.5 mm and a 0.5 inch needle having an internal diameter of 0.012 inches (a 24 gauge needle), an injection force of approximately 100 pounds would be required. However, the injection mechanisms provided in currently available needled autoinjectors are generally not designed to generate such high injection forces.
Subject discomfort is a second hurdle facing the design of a needled autoinjector capable of delivering viscous medicaments. For instance, the sudden application of a force suitable for delivering a viscous medicament via a needle of suitable gauge may startle the subject, particularly if the application of such force causes the transmission of noticeable recoil or impact forces. Thus, an injector capable of delivering viscous medicaments would ideally incorporate a driver that operates without producing sudden, potentially distressing noises or transmitting significant recoil or impact forces to the subject. In addition, driving a needle into the subject with the same force required to drive a viscous medicament through a needle of desired gauge may cause the subject unnecessary physical discomfort. For instance, where the gauge of the needle ranges from about 21 to 31 gauge, an insertion force ranging from about 1 to 7 pounds is believed to be most comfortable. Some studies even suggest that human subjects experience the least amount of pain when the needle is inserted with the least amount of force necessary. Therefore, in order to minimize or reduce the discomfort of the intended subject, an injector capable of injecting viscous medicaments should not only operate unobtrusively, but, where desired, the injector should be capable of inserting a needle with an insertion force tailored to minimize subject discomfort. Ideally, such an injector would generate both an insertion force and an injection force sufficient to deliver a viscous medicament through a needle of desired gauge within a suitable amount of time using a single driving mechanism.